Datasheet

PGA2500 Product Overview
1-4
The common-mode servo function is designed to operate with a total common-
mode input capacitance (including the microphone cable capacitance) of up
to 10nF. Beyond this limit, stable servo operation is not ensured.
The common-mode voltage control input, named V
COM
IN (pin 25), allows the
PGA2500 output and input to be DC biased to a common-mode voltage be-
tween 0V and +2.5V. This allows for a DC-coupled interface between the
PGA2500 preamplifier output and the inputs of common single-supply audio
A/D converters.
A dedicated 0dB input (pin 8) is provided so that the gain of the PGA2500 may
be forced to unity without using the serial port interface. The 0dB input over-
rides gain settings made through the serial port. While the 0dB input is active
(forced high), the serial port register may be updated or data may be passed
through the serial interface to other PGA2500 devices in daisy-chain configu-
ration. However, any changes in gain will not take effect until the 0dB input is
driven low.
The zero-crossing control input, named ZCEN (pin 9), is provided for enabling
and disabling the internal zero-crossing detector function. Forcing the ZCEN
input high enables the function. Zero-crossing detection is used to force gain
changes on zero crossings of the analog input signal. This limits the glitch en-
ergy associated with the switched gain network, thereby minimizing audible
artifacts at the preamplifier output. Since zero-crossing detection can add
some delay when performing gain changes (up to 16ms maximum for a detec-
tor timeout event), there may be cases where the user may wish to disable the
function. Forcing the ZCEN input low disables zero-crossing detection, with
gain changes occurring immediately when programmed.
An overflow indicator output, OVR, is provided at pin 5. The OVR pin is an ac-
tive high, CMOS-logic-level output. The overflow output is forced high when
the preamplifier output voltage exceeds one of two preset thresholds. The
threshold is programmed through the serial port interface using the OL bit. If
OL = 0, then the threshold is set to 5.1Vrms differential, which is approximately
−1dB below the specified output voltage range. If OL = 1, then the threshold
is set to 4.0Vrms differential, which is approximately −3dB below the specified
output voltage range.
The PGA2500 includes four general-purpose programmable digital outputs,
named GPO1 through GPO4 (pins 1 through 4, respectively), which are con-
trolled via the serial port interface. All four pins are CMOS-logic-level outputs.
These pins may be used to control relay drivers or switches used for external
preamplifier functions, including input pads, filtering, polarity reversal, or phan-
tom power.